Wireless Local Area Network (WLAN) technology is becoming widely used in home, office, and in mobile applications. In WLAN systems, modulation techniques such as, for example, 64 Quadrature Amplitude Modulation (QAM) Orthogonal Frequency Division Multiplexing (OFDM) in WLAN 802.11g systems are used for the data transmission. In order for the modulation technique to function reliably, the gain and phase of a Power Amplifier (PA) within the transmitter needs to be substantially linear. A proxy for linearity is Error Vector Magnitude (EVM) expressed in percentage terms. WLAN PAs with an EVM <3% up to a specific power output level are generally considered sufficiently linear to enable the transmission of a full rate 64 QAM with a low packet error rate. Of course, a higher output level from the PA allows for a broader range to the receiving WLAN base station. Therefore, linear power delivery up to the maximum power level before exceeding 3% EVM is sometimes used as a figure of merit for WLAN PAs.
In WLAN systems data is transmitted in bursts. At the start of each burst is a calibration sequence, where the amplitude of the received Radio Frequency (RF) signal is determined for the duration of the burst. The PA RF output signal amplitude depends largely on the stability of the gain of the PA. Unfortunately, the gain of linear PAs tends to increase with increasing supply voltage because the increased supply voltage causes an improvement to the high frequency performance of the underlying semiconductor transistors forming amplification stages within the PA. In most stationary applications of WLAN systems the supply voltage to the PA is sufficiently stable during the burst for this effect to be unnoticeable. However, in the case of WLAN PAs used in battery operated devices—for example, mobile devices such as Global System for Mobile Communication (GSM) cell phones this is not the case. In mobile devices the supply voltage of the PA is directly provided from the battery. Since the output impedance of batteries such as, for example, Li ion batteries, is relatively high, the supply voltage of the PA drops by approximately 400 mV during a data burst and recovers by the same amount after the data burst.
A state-of-the-art solution to this problem is to add a regulator between the battery and the supply voltage to the PA. However, adding the regulator increases complexity and cost, uses valuable space—especially in handheld mobile devices, and reduces overall efficiency of the device.
It would be desirable to overcome at least some of the drawbacks of the prior art.